The present invention relates to a hydraulic control circuit for an automatic transmission with 5 or more forward gear change ratios (shift stages) and more specifically to a hydraulic control circuit including an inhibitor circuit for an automatic transmission.
With respect to a first aspect of the present invention, in a conventional hydraulic control circuit for an automatic transmission with plural gear change ratios, the construction has been such that a shift valve and control solenoid valve are provided for each friction engaging member (referred to as "friction member").
Therefore, in an automatic transmission with multiple gear change ratios such as 5 or more forward gear shift stages; for instance, in a 6-forward-stage automatic transmission provided with 6 friction members of 3 clutches (C1 to C3) and 3 brakes (B1 to B3) and five shift valves (1-2 shift valve, 2-3 shift valve, 3-4 shift valve, 4-5 shift valve, and 5-6 shift valve), a solenoid valve is required for each shift valve to control each shift valve, and therefore the number of solenoid valves increases with increasing shift stages.
In a multiple gear change ratio automatic transmission of 5 or 6 forward gear shift stages, in particular, an increase in the number of the solenoid valves results in an increase in the cost and the space within which these solenoid valves are installed, thus increasing the volume of the automatic transmission.
Further, in the conventional automatic transmission, since a single shift valve is controlled by a single solenoid valve to constitute each gear shift stage as already described, in case the solenoid valve is abnormal, there exists a problem in that the friction members are engaged in duplicate fashion into and out-of-control and the transmission is damaged due to shock generated when the friction members are engaged in duplicated.
With respect to a second aspect of the present invention, in the hydraulic control circuit for an automatic transmission with 5 or more forward gear change ratios, the same friction member is used separately at two different gear shift stages, for instance at the 3 and 5 gear shift stages or the 2 and 6 gear shift stages.
This indicates that hydraulic pressure should be supplied to the same friction member via two different hydraulic passages. Therefore, conventionally change balls are used as change over valves for changing over hydraulic supply passages communicating with the same friction member used separately at two different gear shift stages.
In these change balls, a ball is disposed at a ball seat and hydraulic supply is changed over when the ball is brought into contact with or separated from the ball seat. However, since the ball is readily vibrated on the ball seat whenever hydraulic pressure is introduced or discharged, when used for a long time, the seating tightness between the ball and the seat is degraded, thus raising a problem with durability. The poor durability of the change ball is a serious problem in particular, for trucks which require a long life. Further, there exists another problem such that the change ball is not uniform in quality and therefore poor in reliability.
With respect to a third aspect of the present invention, in the conventional hydraulic control circuits for an automatic transmission, there exist ones provided with a so-called fail-safe function which enables manual travelling in case of solenoid valve disconnection and with an inhibitor function which prevents engine over-run in case of manual gear shift down during high speed travelling.
In the conventional hydraulic control circuit for an automatic transmission, however, there exist some gear shift stages at which the inhibitor function (, i.e., the fail-safe function) is disabled in case of disconnection of the solenoid valve. In other words, in the conventional automatic transmission, there exists a fear such that the engine goes into over-run in case of manual shift down at a high speed.